1. Field of the Invention
The present invention relates to an ion source apparatus used in an ion implanter, particularly, an electronic energy optimized method improving a generating rate of given ions by lowering undesired hydrogen ions, and an ion source apparatus using the method.
2. Related Art
In industries, ion implanting technologies have been generally used for implanting impurities to workpieces such as silicon wafers, glass substrates, etc. when mass-producing products such as integrated circuits, flat panel displays, etc.. Conventional ion implanters have been provided with an ion source, which enables to ionize desired dopant elements and accelerate the elements to form ion beams having normal energy.
The ion source includes one rectangular plasma chamber made of graphite, stainless, aluminum, etc. and an extraction electrode system extracting ions confined in the plasma chamber. The rectangular plasma chamber is constituted of a top wall, 4 side walls and a bottom wall. The plurality of permanent magnets forming a cusped magnetic field for confining plasma are provided at the top wall and the 4 side walls. And, a gas supply opening for the ion source gas and an antenna introduction opening are provided at the top wall while plasma electrodes having opened outlets for extracting ion beams are provided at the bottom wall. The extraction electrode system works to extract the ion beams through electric fields of plasma, and the system is usually formed of the plurality of electrodes such as a plasma electrode, an extraction electrode, a suppression electrode and a ground electrode
The ion source of this kind will extract ions by affecting plasma with electric fields after the ion source gas is made into plasma in the plasma chamber. The ion source gas filled in the plasma chamber is hydrogen compound gas where basic elements of ions combine with hydrogen. For example, PH3 is used for obtaining phosphorus ions while B2H6 is used for obtaining boronic ions. In actual cases, under consideration of easy handling, safety, etc., the hydrogen compound gas is not solely used as the ion source gas, but the ion source gas is diluted with hydrogen gas. Specifically, diborane gas diluted with hydrogen (B2H6/H2), phosphine gas diluted with hydrogen (PH3/H2) or arsine gas diluted with hydrogen (AsH3/H2) are used as the ion source gas.
As described, the ion source gas is the hydrogen compound gas mixed with hydrogen gas. Thus, mixed plasma, that is, plasma based on many kinds of ions, is produced in the plasma chamber. In general, plasma includes not only ions suitable for ion implantation onto workpieces but also ions not suitable therefor. Moreover, plasma includes ions of by-products produced through ionization. Furthermore, Plasma includes electrons having an energy distribution. For example, when using the diborane gas diluted with hydrogen (B2H6/H2), B2Hx+ ions (X=1, 2, 3, 4, 5 and 6) or BHy+ ions (y=1, 2 and 3), etc. are produced in addition to B+, B2+ ions as boron (B)-related ions. Or, H+ ions or H2+, H3+ ions, etc. are produced as hydrogen (H)-related ions. In its ratio, the hydrogen-related ions will be 85% to 15% of the boron-related ions. When using the phosphine gas diluted with hydrogen (PH3/H2) as the ion source gas, the hydrogen-related ions will be 70% to 30% of phosphorus (P)-related ions.
Accordingly, dopant gases such as diborane gas, etc. are diluted with hydrogen gas in the plasma chamber, and high-energy electrons activated in the plasma chamber are generated. Through this ionized processes, hydrogen ions in addition to desired ions (B+ or P+) are produced so as to form ion beams extracting through the opened outlets.
Thus, the hydrogen-related ions are implanted with the desired ions. When the hydrogen-related ions have excess current density, the ions cause undesirable heat increase over the workpieces thereby damaging a silicon wafer or photoresist on a surface of a glass substrate.
In order to decrease number of undesired ions not contributing to the ion beam extraction, the following method is generally known that magnets are provided in the plasma chamber so as to split off ionized plasma. The magnets confine undesired ions and high-energy electrons in a place far from the opened outlets of the plasma chamber whereas they confine desired ions and low-energy electrons in a place nearby the opened outlets of the plasma chamber.
Considering ion doping, etc., there will not have any notable problems if B2Hx+ ions or BHy+ ions extracted from the plasma chamber are included. But, Hx+ ions (X=1, 2 and 3) are not allowed to be included. Those undesired ions cause heat load by colliding with each walls of the plasma chamber, the extraction electrodes or workpieces subject to the ion doping. In addition, accelerated current will be uselessly consumed. Thus, for maintaining high-quality ion sources, it is necessary not to make Hx+ ions included in ion beams.
Accordingly, a magnetic filter is provided in the plasma chamber, specifically, the magnetic filter is provided between a bottom portion of the plasma chamber and the extraction electrodes to be parallel therewith. In this structure, heavy ions advancing toward the extraction electrodes pass through the magnetic filter while light ions are obstructed thereby to pass through. See Japanese patent Application Laid-Open No. Hei 8-209341, pars. 0003-0006 and 0021-0023.
In the ion source apparatus disclosed in the above reference, among ions entered into slits of the magnetic filter, only the light ions such as H+ ions and H2+ ions having small mass are forced to be largely curved by means of magnetic fields formed in the slits orthogonal to advancing directions of those ions. Then, the ions enter into Larmor radius and are trapped by the magnetic fields. Through the above, the light ions on the side of the plasma electrodes will be diffused and disappeared.
On the other hand, the heavy ions such as B+, B2+ ions, B2Hx+ ions, BHy+ ions, P+ ions or PHx+ ions (x=1, 2 and 3) having large mass pass through the slits with little influence in their advancing direction, whereby the ions disperse toward the plasma electrodes. However, the above method will largely and solely depend on performance of the magnetic filter. In addition, the magnetic filter negatively affects production of ions; it works to decrease numbers of the ions. Furthermore, in case various ion source materials accrete to and collect over the magnetic filter, the performance of the filter will be significantly impeded.
Further, Japanese patent Application Laid-Open No. 2000-48734, pars. 0015-0018 shows another method enabling to suppress components ratio of hydrogen ions in ion beams extracted from high-frequency ion sources.
In general, in case cusped magnetic fields formed between high-frequency electrodes and plasma are excessive, it makes possible for high-energy electrons to perform drift motion. And, through the high-energy electrons, hydrogen in high ionization energy is electrolytically dissociated so as to be ionized thus increasing components ratio of hydrogen ions in the ion beams. On the other hand, too weak cusped magnetic fields will inhibit electrons from the drift motion, which makes life of electrons short giving more difficulties to maintain high-frequency discharges.
Therefore, by setting the strength of cusped magnetic fields in sheath formed between plasma in the plasma chamber and the high-frequency electrodes to 1–3 mT, the cusped magnetic fields in the sheath will be controlled. With this structure, the drift motion of the high-energy electrons can be controlled, and the maintenance of the high-frequency discharges in short-life electrons with least drift motion in the sheath can be eased. Accordingly, not only ion beams can be stably extracted, but also the components ratio of hydrogen ions in ion beams can be well repressed.
However, in the above method, because the drift motion of the high-energy electrons needs to be repressed, it makes difficult to produce desired ions from plasma so as to output high-powered ion beams.
Furthermore, a burnous-typed ion source as shown in FIG. 7A can be also used as a standard ion source. Considering the burnous-typed ion source, a pair of magnets 33 is provided so as to face each other and outside an arc chamber 32 provided with plasma electrodes 31 thus generating magnetic fields. Thermoelectrons 36 are discharged from filaments 34 in the arc chamber 32 through cathode caps 35. These thermoelectrons 36 react to boron or phosphorus of ion source gas and generate ions. When a source of electric fields is affected by permanent magnets arranged at interior walls of a plasma chamber, the thermoelectrons 36 are trapped in the electric fields in such a manner as to spirally withdraw along magnetic flux of the external magnets 33 as shown in FIG. 7B. A direction of motion e in the trapped electrons will be determined based on the direction when the thermoelectrons 36 are emitted from the filaments 34. That is, the direction will be equal or inversive to the one of the magnetic flux. This phenomenon is based on cyclotron motion as shown in FIG. 7C, and electrons and ions move along the magnetic flux in such a manner as to revolve about the magnetic flux.
On the other hand, when an RF antenna is arranged in the plasma chamber, desired voltages are applied through high-frequency (or microwave) power. For example, in case a high-frequency electric field of 13.56 MHz is applied to the antenna, an electric field is generated in a direction as shown in FIG. 7D, and also a magnetic field is generated in a direction orthogonal to the direction of the electric field. Thus, electrolytic dissociation will be advanced through collision between electrons accelerated by the electric field and gas molecules thereby producing plasma. However, only through operation of the electric field by the antenna, the drift of electrons is limited. And also, regardless of the generation of the magnetic field, this magnetic field does only have limited influence on the electrons.